It is known how to authenticate original documents using a digital signature. The latter provides a unique pattern based on the digitized document text and a unique signing key (or secret key) of the document creator. When the original with attached digital signature is transmitted (electronically or as a hard copy) to a third party, the latter through digitizing and processing of the received document and digital signature can determine whether the document is genuine.
Ways of providing digital signatures of original documents are described in U.S. Pat. Nos. 4,881,264; 4,309,569; 4,625,076; 4,656,474, and other related patents, whose contents are hereby incorporated by reference. The digital signature, in effect, is an encoded version of the entire document, reduced to a unique marking added as a seal to the original document. If the original document or its signature is modified in any way, the alterations will be detected when the document and signature are checked with the appropriate decoding key.
The first of the above-described patents describes a dual-key or public key cryptosystem for generating a digital signature of a document. In this system, the originator or creator uses a private or secret signing key for processing the document by a particular algorithm to generate a "signature" represented by a sequence of bits ranging from several hundred to several thousand depending upon the particular system used and the level of security desired. The receiver of the digital signature would possess or be given a checking or public key, mathematically related to the signing key, which can be used to process the document through a related algorithm to determine whether the received document was indeed generated with the authentic signing key of the creator. If the contents of the document are to be kept secret, then it can be separately encoded and decoded by one of many known encoding and encryption schemes.
One company has described a so-called time stamp seal (akin to a notary seal) representing a digital signature of the document plus a time stamp to mark the time when the document was received and the digital signature recorded. U.S. Pat. No. 5,001,752 describes in detail another version of a time notarization system.
Another company emphasizes the public key cryptography system as a way of achieving a tamper-proof digital signature that can be used to establish the authenticity of electronic mail messages.
U.S. Pat. No. 4,650,975 describes a scheme for authentication of an identifying IC card employing a personal identification number (PIN) for checking the validity of a card holder using a Data Encryption Standard (DES).
The emphasis in the art has been the authentication of original documents or electronic mail messages. There is a much greater need, however, for a reliable scheme to authenticate a hard copy of an original document. The various schemes described above applicable to original documents would not apply to a copy of the document, or at least would not leave a receiver of such a copy with a high level of confidence that the copy in his possession is, in fact, an exact duplicate of some original document located at a distant place.